Recently, services such as a car telephone, the Internet connection of navigation, an information service, and an emergency reporting system have been commercialized in a mobile such as an automobile.
The frequency bands used for the car telephone are a 0.8 GHz band and a 1.5 GHz or 2 GHz band in Japan, and a 0.8 GHz band and a 1.9 GHz or 2 GHz band in other countries, for example.
For providing these services, an on-vehicle antenna that operates in a plurality of frequency bands in these systems is increasingly required.
The configuration and operation of a conventional monopole antenna that can support three operating frequencies are described with reference to FIG. 8, FIG. 9A and FIG. 9B.
FIG. 8 is a schematic perspective view of the conventional monopole antenna. FIG. 9A and FIG. 9B are characteristic diagrams of the monopole antenna. The monopole antenna 800 includes antenna element 5400 and feeding point 5200 for supplying high-frequency signals to flat conductor 6000 of antenna element 5400.
Antenna element 5400 has flat conductor 6000, resonance circuits 7100 and 7200, linear conductor 5300 of which one end is connected to inner conductor 6100, and. ground plane 5100. Flat conductor 6000 is made of conductive material such as copper, and has inner conductor 6100, first outer conductor 6200, and second outer conductor 6300. Conductors 6100, 6200 and 6300 are formed concentrically from the inside on the same plane. Second outer conductor 6300 has the longest outer diameter D. In flat conductor 6000, the outer edge of inner conductor 6100 is connected to the inner edge of first outer conductor 6200 via resonance circuit 7100, and the outer edge of first outer conductor 6200 is connected to the inner edge of second outer conductor 6300 via resonance circuit 7200.
Resonance circuits 7100 and 7200 are formed so as to provide a resonance frequency set by a parallel circuit of a coil and a capacitor, for example. At this set resonance frequency, the impedance is high. Therefore, in resonance circuit 7100, for example, inner conductor 6100 is insulated from first outer conductor 6200. The impedance is low at a frequency other than the set resonance frequency, so that inner conductor 6100 is substantially electrically connected to first outer conductor 6200. The same is true of resonance circuit 7200.
The other end of linear conductor 5300 connected to flat conductor 6000 of antenna element 5400 penetrates ground plane 5100 and is connected to feeding point 5200. High-frequency signals from a signal source (not shown) are fed to flat conductor 6000 via feeding point 5200 and linear conductor 5300.
In monopole antenna 800 having such a configuration, when highest first frequency f1, intermediate second frequency f2, and lowest third frequency f3 are fed from the signal source to antenna element 5400 via feeding point 5200, antenna element 5400 operates as follows.
Firstly, when first frequency f1 is fed, resonance circuit 7100 has high impedance at first frequency f1 because resonance circuit 7100 is set to resonate with first frequency f1. As a result, inner conductor 6100 is electrically insulated from first outer conductor 6200, and only linear conductor 5300 and inner conductor 6100 resonate.
Next, when second frequency f2 lower than first frequency f1 is fed, resonance circuit 7100 has low impedance. Therefore, inner conductor 6100 is substantially electrically connected to first outer conductor 6200, and second frequency f2 is transmitted to first outer conductor 6200. While, resonance circuit 7200 has high impedance at second frequency f2 because resonance circuit 7200 is set to resonate with second frequency f2. First outer conductor 6200 is, therefore, electrically insulated from second outer conductor 6300. At second frequency f2, not only linear conductor 5300 and inner conductor 6100, but also first outer conductor 6200 resonates.
Next, when third frequency f3 lower than second frequency f2 is fed, resonance circuit 7200 also has low impedance, and first outer conductor 6200 is substantially electrically connected to second outer conductor 6300. As a result, third frequency f3 is transmitted to second outer conductor 6300, and not only linear conductor 5300, inner conductor 6100, and first outer conductor 6200, but also outer conductor 6300 resonates.
Monopole antenna 800 can thus operate at three frequencies. Directivity, namely one of characteristics of monopole antenna 800 is shown in FIG. 9A and FIG. 9B. FIG. 9A and FIG. 9B show characteristics obtained when XYZ orthogonal coordinate system is set using the center of ground plane 5100 as the origin as shown in FIG. 8. FIG. 9A shows the characteristic in the XY coordinates, and FIG. 9B shows the characteristic in the XZ coordinates.
In a typical monopole antenna, the directivity has a circular shape (hereinafter called omni direction) in the XY coordinates and a figure eight shape having right and left shapes that are substantially the same in the XZ coordinates. In the XY coordinates, the radio wave can be transmitted or received longitudinally and laterally in any direction. The figure eight shaped directivity in the XZ coordinates means that dented ellipse is substantially symmetric with respect to the axial line of the Z-axis and the radio wave can be transmitted or received especially in the X-axis direction.
In monopole antenna 800 shown in FIG. 8, the directivities at both second frequency f2 and third frequency f3 have a circular shape in the XY coordinates as shown in FIG. 9A, indicating omni direction. When second frequency f2 and third frequency f3 lie in the 1.9 GHz band on the high frequency side and the 0.9 GHz band on the low frequency side for a car telephone, respectively, for example, the directivity has a circular shape, namely the omni direction, at either frequency.
As shown in FIG. 9B, it is difficult that the directivities at both second frequency f2 and third frequency f3 have a figure eight shape in monopole antenna 800. In FIG. 9B, the directivity at third frequency f3 has a figure eight shape, but the directivity at second frequency f2 has no figure eight shape. The difference between the directivities at second frequency f2 and third frequency f3 in the XZ coordinates in FIG. 9B causes a difference between intensities (hereinafter called radio emission intensities) of the directivities in the XY coordinates in FIG. 9A. In other words, since the directivity at third frequency f3 has the figure eight shape and the directivity at second frequency f2 has no figure eight shape, circles indicating the radio emission intensities at second frequency f2 and third frequency f3 have a different diameter in FIG. 9A. In monopole antenna 800, the radio emission intensity at second frequency f2 is about 3 dBi lower than that at third frequency f3.
A configuration similar to that of conventional monopole antenna 800 is disclosed in Japanese Patent Unexamined Publication No. 2000-059129.
The radio emission intensities at two operating frequencies, namely second frequency f2 and third frequency f3 in the example discussed above, are different from each other in conventional monopole antenna 800. Therefore, when two operating frequencies are required due to a difference in communication company and communication method in a system such as a car telephone, the following problem arises. In other words, required radio emission intensity can be secured and transmitting/receiving sensitivity is high at one frequency, but required radio emission intensity cannot be sufficiently secured and transmitting/receiving sensitivity is low at the other frequency.
The present invention addresses the conventional problem, and provides a monopole antenna that can operate at a plurality of frequencies and can secure required radio emission intensity at any operating frequency.